Π-stacked Structure Research Articles

π-Conjugated polymer-layered structures: synthesis and self-assembly

π-Conjugated polymers capped with a boronic acid end-functional group were synthesized by chain-growth Suzuki–Miyaura polycondensation. The obtained polymers were reacted with a π-stacked-polymer scaffold, developed in our previous work. Despite their bulkiness, the conjugated polymers were efficiently introduced into the polymer scaffold. The spectroscopic and optical characterization of the obtained graft polymers in the solid state revealed that the original π-conjugated polymers were stacked in a single polymer chain. Although excluded volume effects prevented significant intermolecular interactions between the polyfluorenes, the introduced polythiophenes formed intermolecular π-stacked structures. Direct observations of the obtained graft polymers showed that the graft polymers self-assembled into the spherical or fiber structures depending on the introduced polymers, whereas no ordered structures were formed by the same polymers before the polymer reaction. π-Conjugated polymers capped with a boronic acid end-functional group were synthesized by chain-growth Suzuki–Miyaura polycondensation. The obtained polymers were reacted with a π-stacked-polymer scaffold to synthesize graft polymers. The spectroscopic and optical characterization of the obtained graft polymers in the solid state revealed that the original π-conjugated polymers were stacked in a single polymer chain. Direct observations of the obtained graft polymers showed that they self-assembled into spherical or fiber structures depending on the introduced polymers, whereas no ordered structures were formed by the same polymers before the polymer reaction.

Comparison of systematic coarse-graining strategies for soluble conjugated polymers

We assess several systematic coarse-graining approaches by coarse-graining poly(3-hexylthiophene-2,5-diyl) (P3HT), a polymer showing π-stacking of the thiophene rings and lamellar ordering of the π-stacked structures. All coarse-grained force fields are ranked according to their ability of preserving the experimentally known crystalline molecular arrangement of P3HT. The coarse-grained force fields parametrized in the amorphous melt turned out to accurately reproduce the structural quantities of the melt, as well as to preserve the lamellar ordering of the P3HT oligomers in π-stacks. However, the exact crystal structure is not reproduced. The combination of Boltzmann inversion for bonded and iterative Boltzmann inversion with pressure correction for nonbonded degrees of freedom gives the best coarse-grained model.

Halogenated (F, Cl, Br, or I) Diphenylhexatrienes: Crystal Structures, Fluorescence Spectroscopic Properties, and Quantum Chemical Calculations

A series of halogenated compounds, (E,E,E)-1,6-di(4-X-phenyl)hexa-1,3,5-trienes (1: X = F, 2: X = Cl, 3: X = Br, 4: X = I), were synthesized and their crystal structures and fluorescence emission properties were systematically investigated. Single-crystal X-ray analysis reveals that molecules are arranged via X/X halogen bonds and/or CH/X-type hydrogen bonds to form a herringbone structure in 1 and π-stacked structures in 2–4. In the structures of 2 and 3, which are almost isomorphous, the distance and displacement for the nearest stacking molecules are smaller than those in 4. Although the structures of 2–4 are basically not greatly different from each other, the nearest-neighbor arrangements are π-stacked in 2 and 3, but herringbone in 4. Steady-state and time-resolved measurements show that the solid-state fluorescence properties also strongly depend on the halogen size. The fluorescence spectra are red-shifted and the Stokes shifts are large in 2 and 3 relative to those in 1 and 4, resulting from the ...

The Influence of Quasiplanar Structures of Partially Oxygen-Bridged Triphenylamine Dimers on the Properties of Their Bulk Films

Abstract The relationship between molecular orientation and charge-transporting properties in vapor-deposited films of two doubly oxygen-bridged triphenylamine dimers, which differ in their connection mode, was examined. A comparison of the fluorescence spectra obtained from these films, as well as from solution and the crystalline state revealed that the dimers retain two different states in amorphous films, i.e., a monomer-like random orientation and a crystalline-like π-stacking orientation. Regarding the crystalline-like π-stacking orientation in the films, one dimer forms a one-dimensional π-stacking structure, whereas the other dimer forms a two-dimensional π-stacking structure, thus reflecting the difference in packing structure in the crystals. Time-of-flight measurements showed a strong dependence of the charge mobilities in the film of the dimer with a one-dimensional π-stacking structure on the electric field. The film of the dimer with a two-dimensional π-stacking structure exhibited ambipolar charge-transport properties with high electron and hole mobilities. When these dimers were used as hole injection layers in organic light-emitting diode devices, the external quantum efficiencies improved by a factor of 1.2 relative to standard devices.

Experimental and Calculated Spectra of π-Stacked Mild Charge-Transfer Complexes: Jet-Cooled Perylene·(Tetrachloroethene)n, n = 1,2.

The S0 ↔ S1 spectra of the mild charge-transfer (CT) complexes perylene·tetrachloroethene (P·4ClE) and perylene·(tetrachloroethene)2 (P·(4ClE)2) are investigated by two-color resonant two-photon ionization (2C-R2PI) and dispersed fluorescence spectroscopy in supersonic jets. The S0 → S1 vibrationless transitions of P·4ClE and P·(4ClE)2 are shifted by δν = -451 and -858 cm(-1) relative to perylene, translating to excited-state dissociation energy increases of 5.4 and 10.3 kJ/mol, respectively. The red shift is ∼30% larger than that of perylene·trans-1,2-dichloroethene; therefore, the increase in chlorination increases the excited-state stabilization and CT character of the interaction, but the electronic excitation remains largely confined to the perylene moiety. The 2C-R2PI and fluorescence spectra of P·4ClE exhibit strong progressions in the perylene intramolecular twist (1au) vibration (42 cm(-1) in S0 and 55 cm(-1) in S1), signaling that perylene deforms along its twist coordinate upon electronic excitation. The intermolecular stretching (Tz) and internal rotation (Rc) vibrations are weak; therefore, the P·4ClE intermolecular potential energy surface (IPES) changes little during the S0 ↔ S1 transition. The minimum-energy structures and inter- and intramolecular vibrational frequencies of P·4ClE and P·(4ClE)2 are calculated with the dispersion-corrected density functional theory (DFT) methods B97-D3, ωB97X-D, M06, and M06-2X and the spin-consistent-scaled (SCS) variant of the approximate second-order coupled-cluster method, SCS-CC2. All methods predict the global minima to be π-stacked centered coplanar structures with the long axis of tetrachloroethene rotated by τ ≈ 60° relative to the perylene long axis. The calculated binding energies are in the range of -D0 = 28-35 kJ/mol. A second minimum is predicted with τ ≈ 25°, with ∼1 kJ/mol smaller binding energy. Although both monomers are achiral, both the P·4ClE and P·(4ClE)2 complexes are chiral. The best agreement for adiabatic excitation energies and vibrational frequencies is observed for the ωB97X-D and M06-2X DFT methods.

Longitudinal and Lateral Integration of Conducting Polymer Nanowire Arrays via Block-Copolymer-Templated Electropolymerization

Pyrrole (Py) and thiophene derivatives were electropolymerized on a microphase-separated block copolymer [PEO-b-PMA(Az)] coated ITO electrode in order to form the corresponding conducting polymer nanostructures of perpendicularly oriented and hexagonally arranged cylinders as a template without the removal of one of the domains. Polypyrrole (PPy) nanowires grew through PEO cylindrical domains oriented perpendicular to the electrode to afford an array with an aspect ratio of up to 140 and a density of up to 4.4 × 1012 wires/in.2. Long, straight, π-stacked crystalline structures assigned to PPy main chains were observed in high-resolution TEM images of the individual wires. Additional elaborately crafted electropolymerization was demonstrated on a nanometer scale: (i) stepwise electropolymerization of Py and 3,4-ethylenedioxythiophene (EDOT) afforded segmented PPy–poly(3,4-ethylenedioxythiophene) (PEDOT) and PEDOT–PPy wires with nanoheterojunctions and (ii) one-pot electropolymerization of a mixture of Py a...

Molecular orientation of a new anthracene derivative for highly-efficient blue fluorescence OLEDs

A new anthracene derivative of 9,10-bis(2,5-dimethyl-4-(naphthalen-2-yl)phenyl)-2,3-diphenylanthracene (BDNPA) was designed and synthesized as a non-doped blue emitter in organic light emitting diodes (OLEDs). BDNPA has highly rigid structure and thermal stability with decomposition temperature (corresponding to 5% weight loss) of 490°C, because p-naphthyl xylene groups in 9,10-positions were highly twisted to anthracene core due to steric hindrance of xylene groups, and because the 2,3-diphenyl groups were also twisted about 45–49° to anthracene. OLEDs with BDNPA non-doped emitter showed high efficiency of 5.21cd/A due to the carrier mobility with well-aligned π-stacking structure toward out-of-plane by the face-on orientation by grazing incidence X-ray diffraction.

Interchain Helically π-Stacked Assembly of Cationic Chiral Poly(para-phenylene) Derivatives Enforced by Anionic π-Conjugated Molecules through Both Electrostatic and π–π Interactions

Herein, a novel approach for preparing ionic, helically π-stacked polymer assemblies with high dissymmetry factors is presented. We synthesized cationic chiral poly(para-biphenylene) [PPP1] and poly(para-terphenylene) [PPP2] derivatives by introducing alkyl substituents containing both asymmetric centers and quaternary ammonium cations into the side chains of the polymers. We discovered that these polymers have an intrachain helical structure bearing one-handed twisted main chains in solution; however, there was only a slight preference for one twisted conformation over its opposite. Subsequently, by adding an anionic π-conjugated molecule to the cationic polymers, we prepared polymer assemblies bearing an interchain helically π-stacked structure between the polymers and the molecules. Additionally, we clarified that the anionic molecule functioned as the binding species that closed the cationic π-conjugated polymers through both electrostatic and π–π interactions. Although the polymer assembly of PPP1 wa...

π-Stacked structure of thiadiazolo-fused benzotriazinyl radical: Crystal structure and magnetic properties

A novel benzotriazinyl radical with a 2,1,3-thiadiazolo fused ring (1,3-diphenyl-1,2-dihydro-[1,2,5]thiadiazolo[3′,4′:3,4]benzo[1,2-e]-1,2,4-triazine-2-yl; NSNBT) was prepared and characterized by ESR measurement, cyclic voltammetry, and X-ray crystallographic analysis. By a detailed study of bond lengths and angles, it was found that the molecular structure of NSNBT borrows characteristics both from 2,1,3-benzothiadiazole and from the unsubstituted benzotriazinyl radical, and the central phenyl ring presents a phenanthrene-type bond alternation. Molecules were shown to be arranged in a π-stacked columnar structure, with columns connected to each other through sulfur–sulfur interactions in the crystal. It exhibited strong antiferromagnetic interactions (J/kB=−434K) derived from its dimer structure.

Thiophene-based chiral small bandgap π-conjugated polymers: synthesis and optical properties

A series of thiophene-based chiral small bandgap π-conjugated polymers was successfully synthesized. Optical properties of the polymers were investigated by using UV–vis, photoluminescence (PL), circular dichroism (CD) spectroscopies, and specific rotation measurements. Especially, the isothianaphthene–thiophene based copolymers display 10–20 nm longer absorption maximum than those of thiophene–thiophene copolymers in the UV–vis. PL measurements demonstrate that the polymer exhibit light emission. CD spectroscopy measurements reveal helical π-stacking structure of the polymers.

Correction: Ring-opening polymerization of a 2,3-disubstituted oxirane leading to a polyether having a carbonyl–aromatic π-stacked structure

Correction for ‘Ring-opening polymerization of a 2,3-disubstituted oxirane leading to a polyether having a carbonyl–aromatic π-stacked structure’ by Maia Merlani et al., Polym. Chem., 2015, DOI: 10.1039/c4py01711k.

Ring-opening polymerization of a 2,3-disubstituted oxirane leading to a polyether having a carbonyl–aromatic π-stacked structure

Ring-opening polymerization of a 2,3-disubstituted oxirane afforded a polyether exhibiting intramolecular charge-transfer (ICT) interactions due to a carbonyl–aromatic π-stacked conformation.

Characterization of Chloroquine-Ferriheme Solution Structure Using EXAFS and MD

The complex formed between the antimalarial drug chloroquine (CQ) and its target iron(III)protoporphyrin IX (ferriheme) in aqueous solution is of considerable importance in understanding its mechanism of antimalarial activity. Recently, convincing evidence showed that CQ induces the μ-oxo dimeric form of ferriheme in aqueous solution, but the structure of this complex in solution is uncertain.[1] Molecular dynamics (MD) simulations in aqueous solution were used to model the structure of μ-oxo ferriheme and two possible conformations of the CQ-ferriheme complex, one in which CQ π-stacked with the unligated face of ferriheme and the other in which CQ was docked between the porphyrin rings. The EXAFS spectrum of μ-oxo ferriheme obtained from frozen solution strongly supported the hydrated structure determined from MD simulation where an excellent fit to the spectrum was only obtained when incorporating waters of hydration at regions identified by computation. On the other hand, the EXAFS spectrum recorded on the dried solid of μ-oxo ferriheme required no solvating waters to produce excellent agreement with the crystal structure of μ-oxo ferrihaem dimethyl ester.[2] The EXAFS spectrum recorded on a frozen aqueous solution of the CQ-ferriheme complex was able to distinguish between the two conformations modeled computationally. Fits to the EXAFS spectrum using the π-stacked structure produced poor agreement while those obtained using the docked conformation reproduced the spectrum well. This indicated that the latter is the most likely form of the CQ-ferriheme complex in aqueous solution.

The substituent effect on charge transport property of triisopropylsilylethynyl anthracene derivatives

The charge transport property of two triisopropylsilylethynyl anthracene (TIPSAnt) derivatives TIPSAntBt and TIPSAntNa (bithiophene and naphthalene are introduced at the 2, 6-positions of the TIPSAnt core) were explored through quantum chemical method. To gain a better understanding of the substituent effect on the charge transport property, the results of the parent molecule TIPSAnt was also provided here for comparison. The substituent effect on the molecular geometry, reorganization energy, frontier orbitals, ionization potential (IP) and electronic affinity (EA), crystal property, transfer integrals and charge mobility, band structure and effective mass of the two compounds were investigated to establish the relationship between structures and properties. The introduced bulky TIPS groups made the two compounds adopt two-dimensional, face-to-face, π-stacking structures. The efficient overlaps of π-orbital and smaller π-stacking distance are proved to be the main reason for the high charge mobility of TIPSAntBt and TIPSAntNa. The hole mobilities of TIPSAntBt and TIPSAntNa are 0.88 and 3.60cm2V−1s−1, respectively, which is well consistent with experiment values (0.2 and 3.7cm2V−1s−1, respectively). For TIPSAntBt, the electron mobility (1.29cm2V−1s−1) is a little higher than that of hole due to the more effective transfer integrals of electron. On the contrary, the hole mobility of TIPSAntNa is 20 times larger than that of electron because of the smaller reorganization energy and larger transfer integral of hole, indicating that TIPSAntNa could be used as p-type semiconductor. For TIPSAntBt, the transfer integral is smaller than the reorganization energy, so the hopping mechanism plays a key role in the charge transport property. While the bandwidths and effective mass of TIPSAntNa agreed well with the calculated transfer integrals and charge mobility results. The introduced small substituents to TIPSAnt core contributed to the dramatically different charge transport property from an n-type semiconductor of TIPSAntBt to p-type semiconductor of TIPSAntNa, which shed light on molecular design for an n-type semiconductor through simple chemical structural modification.

Folded discotic dimers.

We report the synthesis and properties of a dibenzophenazine discotic dimer, which was found to adopt a folded conformation in solution. This stable π-stacked structure persists at elevated temperatures and in all solvents examined. Folding allows the dimer to assemble into a columnar liquid crystal phase, despite its short linking group.

Heat, pressure and light-induced interconversion of bisdithiazolyl radicals and dimers.

The heterocyclic bisdithiazolyl radical 1b (R1 = Me, R2 = F) crystallizes in two phases. The α-phase, space group P2₁/n, contains two radicals in the asymmetric unit, both of which adopt slipped π-stack structures. The β-phase, space group P2₁/c, consists of cross-braced π-stacked arrays of dimers in which the radicals are linked laterally by hypervalent 4-center 6-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility measurements on α-1b indicate Curie-Weiss behavior (with Θ = -14.9 K), while the dimer phase β-1b is diamagnetic, showing no indication of thermal dissociation below 400 K. High-pressure crystallographic measurements indicate that the cross-braced π-stacked arrays of dimers undergo a wine-rack compression, but the dimer remains intact up to 8 GPa (at ambient temperature). The resistance of β-1b to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer β-1a (R1 = Et, R2 = F), which readily dissociates into a pair of radicals at 0.8 GPa. The different response of the two dimers to pressure has been rationalized in terms of differences in their linear compressibilities occasioned by changes in the degree of cross-bracing of the π-stacks. Dissociation of both dimers can be effected by irradiation with visible (λ = 650 nm) light; the transformation has been monitored by optical spectroscopy, magnetic susceptibility measurements, and single crystal X-ray diffraction. The photoinduced radical pairs persist up to temperatures of 150 K (β-1b) and 242 K (β-1a) before reverting to the dimer state. Variable-temperature optical measurements on β-1b and β-1a have afforded Arrhenius activation energies of 8.3 and 19.6 kcal mol(-1), respectively, for the radical-to-dimer reconversion. DFT and CAS-SCF calculations have been used to probe the ground and excited electronic state structures of the dimer and radical pair. The results support the interpretation that the ground-state interconversion of the dimer and radical forms of β-1a and β-1b is symmetry forbidden, while the photochemical transformation is symmetry allowed.

Disilanyl double-pillared bisternaphthyl (SiDPBT): synthesis and interfused packing structures with herringbone and π-stack motifs.

A silacyclophane molecule with two disilanyl pillars and two oligoarylene units was synthesized. The molecule was packed in a single crystal with a new motif interfusing π-stack and herringbone packing structures. The hole transporting ability of the solid was revealed by using the flash-photolysis time-resolved microwave conductivity method. The molecular structure, albeit a singly-bonded arylene macrocycle, was rigidified by the unique packing array, which favorably contributed to the hole transporting ability of the solid via the small reorganization energy through the charge transport.

Synthesis of methoxy-substituted picenes: substitution position effect on their electronic and single-crystal structures.

A series of picenes having methoxy groups was synthesized through Pd-catalyzed Suzuki-Miyaura couplings or Wittig reaction/intramolecular cyclization sequences, and their physicochemical properties and single-crystal structures were evaluated. The substitution position effects between the outer 1,12-, 2,11-, and 4,9-position and the inner 3,10-position are quite different; the former showed the same electronic structure as that of picene, but the latter results in a HOMO geometry different from those of picene and other methoxy picenes. In addition, crystal structures of four types of methoxy-substituted picenes 4a-c,e strongly depend on their substitution position and number of methoxy groups, which dramatically changes the structures from the fully anisotropic 1D π-stacked structure to a unique 3D herringbone structure due to steric hindrance of methoxy groups. The calculations of transfer integrals based on their single-crystal structures reveal that the methoxy picenes have intermolecular overlaps less effective than that of the parent nonsubstituted picene. These results are attributed not only to the packing structure but also to electronic structures such as the HOMO distribution. The preliminary OFET of the representative 4c,e showed hole mobilities significantly lower than that of picene due to their less effective intermolecular overlaps, as predicted by the calculated transfer integrals.

Synthesis and solid-state structures of a tetrathiafulvalene-conjugated bistetracene.

A tetrathiafulvalene (TTF)-conjugated bistetracene was synthesized and characterized in the molecular electronic structures based on the spectroscopic measurements and the single-crystal X-ray diffraction analysis. UV/Vis absorption and electrochemical measurements of 5 revealed the considerable electronic communication between two tetracenedithiole units by through-bond and/or through-space interactions. The difference in the crystal-packing structures of 5, showing polymorphism, results in a variety of intermolecular electronic-coupling pattern. Of these, the π-stacking structure of 5 A gave a large transfer integral of HOMOs (97 meV), which value is beyond hexacene and rubrene, thus, quite beneficial to achieve the high hole mobility.

Self-Aggregation and Optical Absorption of Stilbazolium Merocyanine in Chloroform

Dipolar aggregation is in many cases detrimental for the functioning of optical materials. In this study we investigate self-aggregation and optical absorption of stilbazolium merocyanine (SM) in chloroform solution by performing classical Molecular Dynamics (MD) simulations under ambient conditions. The reversal solvatochromic shift, the large bathochromic shift, and the structured absorption band presented by SM in chloroform solution are all aspects of its optical absorption behavior for which the existence of self-aggregation is yet not completely understood. Moreover, the spectroscopic properties of SM oligomers and their occurrence in solvent of low polarity remain a relevant topic that deserves to be investigated. Our analysis of the aggregation behavior of SM in chloroform verified that the majority of the chromophores are involved in the formation of oligomers in solution, where the whole dimer and part of the trimer populations present a stable π-stacking structure. The optical properties of the monomers and oligomers in solution were evaluated by means of a discrete polarizable embedding quantum mechanical/molecular mechanical (PE-QM/MM) response scheme where the quantum part is described at the level of density functional theory. The visible absorption spectrum of SM in chloroform is simulated using time average values obtained for the monomeric and oligomeric forms of SM from the PE-QM/MM calculations performed on uncorrelated configurations extracted from the classical MD simulations. This study shows that the self-aggregation of SM in chloroform may exist, but it is not essential for reproducing the reversal solvatochromic shift in chloroform and that the process does not contribute to enhance the bathochromic shift nor explain the structure observed in its absorption band. Moreover, it is verified that since the electronic transitions of the monomer and oligomers are close together, changes in the interplane separation between the monomeric units of the stacked oligomers substantially affect the spectral resolution of their contribution to the optical absorption spectrum.